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Physics Chat

Phonon-induced dephasing of a non-linear response in a quantum dot-cavity system

Speaker: Liubov Sirkina (EMuljarov)
Date: Friday 5 November 2021
Time: 15:00
Venue: Zoom

Phonons have significant effect on exciton coherent dynamics in semiconductor quantum dots (QDs), demonstrating a remarkable non-Markovian behavior [1]. This was shown by a recent theoretical work for a linear optical response [2] of a strongly coupled QD-microcavity system. While the latter is hard to measure experimentally, nonlinear optical polarisation can be reliably studied using heterodyne spectral interferometry [3]. The four wave mixing (FWM) response of a QD-cavity system has been a focus of recent theoretical works but the exciton-phonon interaction has either been neglected [3] or treated perturbatively [4]. We develop an exact semi-analytic approach to the FWM polarisation of a QD-cavity system coupled to longitudinal acoustic phonons. Our approach is a generalisation of the method [2], employing the Trotter decomposition with linked cluster expansion to account for the exciton-phonon and exciton-cavity couplings on equal footing. Although the treatment of optical nonlinearities increases the complexity of the approach, it remains physically intuitive and computationally straightforward. We find, for a relatively small exciton-cavity coupling strength g=0.035 meV in [3], our exact FWM spectra are qualitatively similar to those simulated [3] without phonons. The effect of phonons is concentrated mainly in the polaron shift of the exciton frequency and a reduction of g by the Huang-Rhys factor [2]. Our work generalises this result, which we refer to as a polaron model, to an arbitrary optical nonlinearity. As g becomes larger (g=0.3 meV in [5]), the FWM shows a spectral asymmetry and a significant deviation from the polaron model. This becomes even more pronounced at elevated temperatures, showing a very striking difference at T=50K. In this regime, the phonon cloud around the QD is unable to adiabatically adapt to a varying optical state, resulting in a non-Markovian dynamics and phonon-assisted transitions between states of the Jaynes-Cummings ladder. [1] B. Krummheuer, V. M. Axt, and T. Kuhn, Phys. Rev. B 65, 195313 (2002). [2] A. Morreau and E. A. Muljarov, Phys. Rev. B 100, 115309 (2019). [3] J. Kasprzak et al., Nat. Matter. 9, 304 (2010). [4] D. Groll et al., Phys. Rev. B 101, 245301 (2020). [5] C. Dory et al., Scientific reports 6, 1 (2016).

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